1. Field of Invention
The present invention relates to a composite optical film. More particularly, the present invention relates to a composite optical film with a low refractive index. The composite optical film can be used in fabrication of a laser reflection lens.
2. Description of Related Art
A laser beam has characteristics of high directionality, monochromaticity, and high brightness. Therefore, the laser beam is widely used, for example in alignment, measurement of distance, thickness, and velocity. The application fields of a laser beam further comprise welding, coevaporation, puncturing, cutting, and optical communication, laser discs, and compact disks. A laser beam can even be applied in medical instruments or equipment. In the practical application, the coating quality of a laser reflection lens is a crucial factor that affects accuracy of a laser device.
A laser reflection lens comprises optical thin films with high refractive index and optical films with low refractive index alternately stacked on a quartz glass substrate. For an argon fluoride (ArF) or a krypton fluoride (KrF) excimer laser device, calcium fluoride (CaF.sub.2) is commonly used as a coating material for ultra-violet region. However, a CaF.sub.2 film prepared by current coating technique such as alternating magnetic sputtering method and electron-beam coevaporation, poor physical properties and inhlomogeneoity result. The disadvantages include low hardness, porosity, and poor adhesion to the quartz glass substrates.